A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, hearing prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system, including an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110.
Typically, the electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104. Depending on context, the electrode contacts 112 are also referred to as electrode channels. In cochlear implants today, a relatively small number of electrode channels are each associated with relatively broad frequency bands, with each electrode contact 112 addressing a group of neurons with an electric stimulation pulse having a charge that is derived from the instantaneous amplitude of the signal envelope within that frequency band.
FIG. 2 shows various functional blocks in a signal processing arrangement for producing electrode stimulation signals to electrode contacts in an implanted cochlear implant array according to a typical hearing implant system. A pseudo code example of such an arrangement can be set forth as:
Input Signal Preprocessing:BandPassFilter (input_sound, band_pass_signals)Envelope Extraction:BandPassEnvelope (band_pass_signals, band_pass_envelopes)Stimulation Timing Generation:TimingGenerate (band_pass_signals, stim_timing)Pulse Generation:PulseGenerate (band_pass_envelopes, stim_timing, out_pulses)The details of such an arrangement are set forth in the following discussion.
In the arrangement shown in FIG. 2, the initial input sound signal is produced by one or more sensing microphones, which may be omnidirectional and/or directional. Preprocessor Filter Bank 201 pre-processes this input sound signal with a bank of multiple parallel band pass filters (e.g. Infinite Impulse Response (IIR) or Finite Impulse Response (FIR)), each of which is associated with a specific band of audio frequencies, for example, using a filter bank with 12 digital Butterworth band pass filters of 6th order, Infinite Impulse Response (IIR) type, so that the acoustic audio signal is filtered into some K band pass signals, U1 to UK where each signal corresponds to the band of frequencies for one of the band pass filters. Each output of sufficiently narrow CIS band pass filters for a voiced speech input signal may roughly be regarded as a sinusoid at the center frequency of the band pass filter which is modulated by the envelope signal. This is also due to the quality factor (Q≈3) of the filters. In case of a voiced speech segment, this envelope is approximately periodic, and the repetition rate is equal to the pitch frequency. Alternatively and without limitation, the Preprocessor Filter Bank 201 may be implemented based on use of a fast Fourier transform (FFT) or a short-time Fourier transform (STFT). Based on the tonotopic organization of the cochlea, each electrode contact in the scala tympani typically is associated with a specific band pass filter of the Preprocessor Filter Bank 201. The Preprocessor Filter Bank 201 also may perform other initial signal processing functions such as and without limitation automatic gain control (AGC) and/or noise reduction and/or wind noise reduction and/or beamforming and other well-known signal enhancement functions.
FIG. 3 shows an example of a short time period of an input speech signal from a sensing microphone, and FIG. 4 shows the microphone signal decomposed by band-pass filtering by a bank of filters. An example of pseudocode for an infinite impulse response (IIR) filter bank based on a direct form II transposed structure is given by Fontaine et al., Brian Hears: Online Auditory Processing Using Vectorization Over Channels, Frontiers in Neuroinformatics, 2011; incorporated herein by reference in its entirety.
The band pass signals U1 to UK (which can also be thought of as electrode channels) are output to an Envelope Detector 202 and Fine Structure Detector 203. The Envelope Detector 202 extracts characteristic envelope signals outputs Y1, . . . , YK that represent the channel-specific band pass envelopes. The envelope extraction can be represented by Yk=LP(|Uk|), where |.| denotes the absolute value and LP(.) is a low-pass filter; for example, using 12 rectifiers and 12 digital Butterworth low pass filters of 2nd order, IIR-type. Alternatively, the Envelope Detector 202 may extract the Hilbert envelope, if the band pass signals U1, . . . , UK are generated by orthogonal filters.
The Fine Structure Detector 203 functions to obtain smooth and robust estimates of the instantaneous frequencies in the signal channels, processing selected temporal fine structure features of the band pass signals U1, . . . , UK to generate stimulation timing signals X1, . . . , XK. The band pass signals U1, . . . , Uk can be assumed to be real valued signals, so in the specific case of an analytic orthogonal filter bank, the Fine Structure Detector 203 considers only the real valued part of Uk. The Fine Structure Detector 203 is formed of K independent, equally-structured parallel sub-modules.
The extracted band-pass signal envelopes Y1, . . . , YK from the Envelope Detector 202, and the stimulation timing signals X1, . . . , XK from the Fine Structure Detector 203 are input signals to a Pulse Generator 204 that produces the electrode stimulation signals Z for the electrode contacts in the implanted electrode array 205. The Pulse Generator 204 applies a patient-specific mapping function—for example, using instantaneous nonlinear compression of the envelope signal (map law)—That is adapted to the needs of the individual cochlear implant user during fitting of the implant in order to achieve natural loudness growth. The Pulse Generator 204 may apply logarithmic function with a form-factor C as a loudness mapping function, which typically is identical across all the band pass analysis channels. In different systems, different specific loudness mapping functions other than a logarithmic function may be used, with just one identical function is applied to all channels or one individual function for each channel to produce the electrode stimulation signals. The electrode stimulation signals typically are a set of symmetrical biphasic current pulses.
While the foregoing discussion of cochlear implant systems covers many persons suffering from impaired hearing, some patients suffer from growth of a tumor in close vicinity to the auditory nerve; e.g. in patients with Neurofibromatosis Type II (NF2). This tumor has to be surgically removed, and in most cases, the auditory nerve also is removed together with the tumor. In some such cases, the auditory nerve may remain partially intact, but within a period of time after surgery, it loses the ability to transmit action potentials elicited in the cochlear to the brainstem. Consequently, such patients are either deaf immediately after the surgery, or they become deaf some time later. For many such patients, hearing can be restored by an auditory brainstem implant (ABI).
The main structural difference between a cochlear implant (CI) system and an ABI system is in the form of the implanted electrode array. FIG. 5 shows an example of an ABI electrode array 500 which has an electrode lead 502 containing wires that deliver the electrical stimulation signals to electrode contacts 503 and a reference contact 504 on an electrode paddle 501 that is located at the distal end of the electrode lead 502. A polyester paddle mesh 505 supports the electrode paddle 501 which is placed against the brainstem of the implanted patient.
While ABI systems have proven to be a great help providing hearing perception for the patients who receive them, it is well known that they suffer from some shortcomings as compared to cochlear implants. For example, tonotopicity in the brainstem is less pronounced than in the cochlea. As a result, much more effort and time (longer surgery) is needed to find an optimal location for the electrode contacts on the ABI electrode array. Moreover, while the tonotopicity in the cochlea is one-dimensional along the longitudinal extension of its scalae, in the brainstem the tonotopicity is distributed over a two-dimensional area (at least, it may be even considered over a three-dimensional space region in the brainstem because the addressable brainstem region for this purpose is not only at the surface).
At present, a good placement of the ABI electrode array is based on electrically evoked auditory brainstem responses (EABR) that are measured during the surgery (when the patient is not able to give subjective feedback because of the anesthesia). However, EABR measurements are not frequency specific, and therefore, they provide only limited help with the placement of the electrode array. Because of the foregoing, it is often the case that hearing perception by ABI patients is usually worse than for CI patients.
Auditory sensing via the cochlea is an example of an afferent nerve sensing pathway. Afferent neurons act as sensing mechanisms that direct sensing signals from different parts of the body towards the brain, providing the brain with information about the condition of that body location. By contrast, efferent nerve pathways operate in the other direction, from the brain to a remote body location, to initiate some action. There is an efferent nerve path from the brain to the cochlea, known as the olivocochlear system. These efferent neurons terminate at the inner (medial) hair cells and the outer (lateral) hair cells within the cochlea to create mechanical action. Commonly understood functions of this arrangement include protection against loud sounds, improving signal to noise ratio, frequency selectivity, and processing of interaural time and phase differences.
Otoacoustic emissions are sounds that originate in the cochlea due to motion of the sensory hair cells within the cochlea as they respond to auditory stimulation. Otoacoustic emissions propagate out from the oval window membrane through the middle ear and across the tympanic membrane into the ear canal. Otoacoustic emissions are detectable by an sensitive microphone placed in the middle ear or ear canal, and they currently are used as the basis testing for hearing defects in newborn babies.